Vulnerability to hydraulic dysfunction as affected by sowing date in rice leaves

Hydraulic properties of leaf blade were studied to provide basic knowledge to examine relationships between leaf vulnerability to hydraulic dysfunction due to low water potential (Ψ) and plant adaptation to water stress in rice. The objectives were to find the appropriate portion along a leaf blade for measurements of hydraulic dysfunction due to low Ψ and to examine whether leaf vulnerability to hydraulic dysfunction changed with the growing season. The early cultivar Bouzu (to supply materials earlier) and Koshihikari (a leading cultivar of Japan) were grown in pots. Hydraulic profile along a leaf blade was determined under well-watered and drought conditions in the two cultivars. The vulnerability to leaf hydraulic dysfunction was compared between plants sown at different dates only in Koshihikari. In both cultivars, leaf hydraulic conductivity (K) was constant in the basal half of a leaf blade, whereas it became smaller toward the tip and this pattern was maintained under drought conditions, indicating that the basal half was an appropriate portion for estimation of leaf hydraulic dysfunction. In Koshihikari, K decreased as relative water content (RWC) decreased and RWC at which K was reduced to 50% of the maximum (RWC_K50) was determined for each sowing date. Then leaf Ψ corresponding to RWC_K50 (Ψ_K50) was estimated from the relationship between Ψ and RWC. As a sowing date was delayed from May to August, Ψ_K50 increased from -1.44 MPa to -1.13 MPa. It was concluded that rice leaves became vulnerable to hydraulic dysfunction due to low Ψ as the growing season was delayed.